Concept: The stability of an element is often related to its electronic configuration, particularly achieving a noble gas configuration (a full outer shell). For elements with a complete outer shell, they exist as stable monatomic entities.

Why (A) is correct:

The electronic configuration of element A is $1s^{2}2s^{2}2p^6$. This configuration shows that the outermost shell (n=2) has 8 electrons ($2s^{2}2p^6$), which is a complete octet. This is the electronic configuration of Neon (Ne), which is a noble gas. Noble gases are highly stable and exist as monatomic molecules (single atoms) because their electron shells are already full, and they have no tendency to gain, lose, or share electrons to form bonds with other atoms of the same element.

Option Analysis:

• A) $A$: Correct. As explained above, element A is a noble gas with a stable octet, so it exists as a single atom.
• B) $A_2$: Incorrect. Noble gases do not form diatomic molecules because they are already stable as single atoms.
• C) $A_3$: Incorrect. Noble gases do not form triatomic molecules.
• D) $A_4$: Incorrect. Noble gases do not form polyatomic molecules.

Correct Answer: (A)